The present invention relates to a technique for reducing the influence of noise generated in a functional block on other functional blocks in a semiconductor integrated circuit (IC) device, and more particularly to a semiconductor IC device adapted to balance the protection against noise interference with the protection against electrostatic breakdown, i.e., surge.
Late years, in semiconductor IC devices, a noise level has become increasingly higher due to increase in operating frequency associated with progress in higher processing speed. Under such a situation, there has been proposed a technique of reducing noise interference between a plurality of functional blocks (each hereinafter referred to simply as “block”) based on isolation of a pad. FIG. 8 shows one example of this type of technique. In this semiconductor IC device, a lead frame 801 and a pad 803 are electrically connected to each other through a wire 802. The pad 803 is connected to a plurality of blocks through respective intra-chip wires 804.
Specifically, as shown in FIG. 8, in order to prevent noise occurring in one of three blocks A to C from interfering with other two blocks, the three blocks A to C are connected at a single point in or near the pad 803. In this technique, if the single connection point has a sufficiently low impedance, noise interference can be reduced as expected. In reality, elements, such as the wire 802 and the lead frame 801, in a terminal area between the pad 803 and the outside of a semiconductor package, have a certain level of impedance, and it is difficult to adequately lower the impedance at the single connection point, which is liable to cause the problem of noise interference between the blocks.
There has therefore been proposed another technique of more effectively preventing noise interference, as disclosed in R. Jacob Baker, Harry W. Li, and David E. Boyce, “CMOS Circuit Design, Layout, and Simulation”, ISBN 0-7803-3416-7 (referred to as “Publication 1” hereinafter). This technique will be described below with reference to FIG. 9.
As shown in FIG. 9, a wire to be connected to a lead frame 801 is divided into three wires 121, 122, 125 which are connected, respectively, to three pads 131, 132, 135, and isolated from each other. The pads 131, 132, 135 are not connected to each other in a wiring layer on a semiconductor substrate but through the wires. Specifically, a position for connecting the wires at a single point is set on the lead frame 801. Thus, as compared with the technique illustrated in FIG. 8, an impedance value at the single connection point can be lowered to more effectively reduce the noise interference.
In the conventional technique illustrated in FIG. 9, each functional terminal, such as a source terminal or a ground terminal, is provided in a number of only one, and there is no specific problem as long as a current supplied from the terminal falls within an allowable range. However, if the current supplied from the terminal is increased beyond the allowable range, or a plurality of terminals are provided for the purpose of noise reduction, the following problem will occur. If the technique illustrated in FIG. 9 is used for reducing noise interference under the above conditions, electrostatic breakdown level will be undesirably lowered due to the wires which are not connected together in a wiring layer, although noise interference can be reduced.